This change adds another check to the debug line table parsing code, namely to make sure the final offset after parsing a table matches the end offset as expected based on the unit length field. These two won't match if the unit length points to part way through an opcode, as the parsing does not stop midway through opcodes.
If the problem is detected, the offset will be reset to the expected value and an error reported.
I guess that the comment is not 100% accurate. Let's imagine the following purely illustrative and meaningless sequence at the end of the section: 0, 5, DW_LNE_set_address, 0, 1, DW_LNE_end_sequence. The extractor will not read an argument of DW_LNE_set_address and will not increment the offset, but after that, something which looks like a correct termination will be read.